1. Field of the Invention
This invention relates to an arrangement for determining the length of shift registers and, more particularly, to an arrangement adapted to make such a determination where the shift register is of an unknown, arbitrary length not exceeding a known maximum in connection with the testing of large scale integrated chip logic circuitry.
2. Description of the Prior Art
For the precise execution of write and read operations of shift registers, it is necessary to know the exact length of such shift registers. The use of shift registers is sufficiently known and has played an increasingly important role in connection with the testability of logic circuits produced at maximum integration density on semiconductor chips (LSI chips).
The inputs and outputs of such logic circuits are in each case connected to latch circuits in which their input and output information can be stored. Because of the notorious lack of connections to the outside world, such input and output latch circuits are connected in the form of shift registers on a semiconductor chip, whereby one such latch circuit forms one stage of a shift register. The number of latch circuits on a particular semiconductor chip, and thus the length of the shift register, differ greatly and are not subject to any law.
During normal operation of the circuits on the semiconductor chip, the input and output latch circuits are, as a rule, not connected in the form of shift registers; this being generally the case only during testing. Thus, a shift register tool permitting arbitrary manipulations of such shift registers is needed both for testing newly developed circuits and for error searching installed systems. Essentially, two basic functions are required; reading of shift registers and displaying the contents stored in them and writing (or setting) freely selectable patterns into such shift registers.
As it is a prerequisite of such shift register manipulations to know the length of the shift register, it has to be indicated either before shifting how long the shift register is or fixed lengths for particular shift registers have to be provided in the shift register tool. Whereas the former approach, in addition to being inconvenient, entails the risk of faulty operation, the latter approach necessitates that each change in the length of a shift register be accompanied by a simultaneous change of the shift register tool. The disadvantage of this is an extraordinary degree of inflexibility in the case of the above-mentioned applications, for which, as is known, shift registers of unknown and considerably differing lengths are formed on various semiconductor chips.